The nonobese diabetic (NOD) mouse spontaneously develops autoimmune type 1 diabetes (T1D), which appears to be dependent upon a balance between pathogenic and regulatory T cells. The genetics and pathogenesis of diabetes are very complex and few of the genes involved in disease have yet been identified. Various defects have been reported in T lineage cells from NOD mice including immature and mature alpha-beta-TCR+ cells, NKT and NK cells, all of which share common T cell precursors. Deviations in early T cell development may have far-reaching affects on future T cell lineages and functions. We have recently shown that even Rag(null) and wild type NOD TCR-negative pre-T cells exhibit defects in their developmental programming. At 5-7 weeks of age, NOD-scid and -Rag(null) mice no longer arrest normally at the beta-selection checkpoint and CD4+ and CD4+CD8+ cells spontaneously appear despite an absence of TCRbeta, is normally required for pre-TCR signaling at this stage. Even prior to the checkpoint violation, derangements were observed in the expression of several cell surface receptors that have known effects on TCR signaling. Thus, immunodeficiency revealed a major defect in pre-T cell development and checkpoint control. Preliminary linkage mapping data show a significant association between this trait and an important cluster of diabetes susceptibility loci, Idd9, on chromosome 4. Wild type NOD pre-T cells also exhibit some similar derangements in pre-T cell surface receptor expression levels and undergo abnormal differentiation in vitro which may be linked to the NOD-Rag(null) trait. Because this genetic defect in early T cells precedes divergence of all T cell lineages, as well as diabetes pathogenesis, it may provide unique access to a subset of diabetes susceptibility genes central to T cell differentiation and function. Towards defining the cellular, genetic and molecular mechanisms causing this checkpoint violation, we propose to (1) determine if the defect is intrinsic to the T cell precursors or the thymic environment, and find the source of the abrupt change in development at 5-7 wks of age, (2) determine the relationship between wild type and Rag(null) NOD pre-T cell abnormalities and characterize their developmental consequences, and (3) determine if the beta-selection checkpoint violation maps to Idd9 and/or other known Idd loci, using congenic mice and linkage mapping. This long-term aim of the proposed study is to understand the nature of a defect in NOD early T cell development, which maps to a major diabetes susceptibility genetic region, and identify the predisposing gene(s) and affected pathways. Identifying genes and pathways affecting T cells, which predispose NOD mice to T1D will help in the understanding of immune deviations that can lead to disease in humans.